drm/amdgpu: Init pcie_index/data address as fallback (v2)

To allow using this helper for indirect access when
nbio funcs is not available. For instance, in ip
discovery phase.

v2: define macro for pcie_index/data/index_hi fallback.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>